Corrosion prevention device for irrigation pipe



May 20, 1969 R. M. SHERMAN 3,445,370

CORROSION PREVENTION DEVICE FOR IRRIGATION PIPE Fild May 7, 1965INVENTOR. ROGER M. SHERMAN ATTORNEYS United States Patent U.S. Cl.204197 4 Claims ABSTRACT OF THE DISCLOSURE A cathodic system forpreventing the corrosion of aluminum pipe including a sacrificial zincwire located within the pipe and secured by anchoring means at the endsof the wire to the inside pipe wall.

This invention relates to a cathodic protection device for combating andinhibiting corrosion in piping.

Metallic structures installed underground, on the ground, in water, orotherwise in contact with an electrolytic medium are subject tocorrosion due to a chemical reaction with non-metallic elements in thesurrounding medium. The amount or degree of corrosion that takes placemay be influenced by several factors such as the type of metal in thestructure, the amount of impurities in the metal and the chemical natureof the nonmetallic medium.

Irrigation piping heretofore provided a particularly difficult corrosionproblem because it is generally formed of relatively thin-walledaluminum material. This enables such pipe to be light and readilymovable from place to place in conjunction with sprinkler systems.However, even a relatively small amount of corrosion on such piping canbe sufficient to cause leakage and a severe weakening of the pipe.

When metal corrodes, it replaces hydrogen or another metal from acompound in the surrounding medium. The reactions take placesimultaneously on the metal surface. In one reaction metal ions passinto solution and in the other hydrogen ions pass out of solution toform hydrogen gas. Since these two reactions cannot take place at thesame point, there must be two kinds of areas on the metal surface: thosewhich are anodic where metal dissolves, and those which are cathodicwhere hydrogen ions are discharged. Thus, for corrosion to take place ametal structure must have these dissimilar areas, and the interfacebetween a metal and its environment cannot be entirely homogenous. Thisprinciple is borne out by the fact that pure, almost perfectlyhomogeneous, metals have an extremely high resistance to corrosion, evenwhen exposed to highly acidic solutions.

In the normal production of aluminum irrigation pipe, the presence ofmetallic impurities, oxides and differences in crystal structure andphysical strain cannot be avoided and therefore perfectly homogeneousmaterial is not commercially available for use as irrigation pipe. Ithas been shown that two areas of different properties on the surface ofa metallic structure will have different potentials and therefore theiradjacent areas will form galvanic cells comprised of an anode area whichdissolves metal into the solution and a cathode area that dischargeshydrogen ions. There is a direct relationship between the quantity ofcurrent which passes through these cells and the amount of metal whichis dissolved.

The basis for any electro-chemical treatment of corrosion is theelectro-motive force series which is an arrangement of the elements inorder of their dissolution tendencies. It must be recognized that thisseries is dependent on the electrolyte in use at the time ofmeasurement. In

3,445,370 Patented May 20, 1969 ice other words, it may be said thatevery solution has its own electro-motive force series.

Water that flows through most irrigation piping systems usually hasimpurities containing various ions, and it therefore acts as anelectrolyte Corrosive action may therefore take place on the insidesurface of the pipe as described above when any One portion of themetallic pipe structure becomes anodic with respect to another part ofthe same pipe or with respect to some other object in the surroundingmedium. During this electrolytic corrosive action metal in the anodicareas of the structure is electrochemically removed, and a weakening andeventually perforation of the piping results. It is wel known thatcorrosive structures can be protected by attaching an expandablematerial that is anodic with respect to the structure so that materialwill be dissolved from the sacrificial anode rather than from thestructure itself. However, various attempts to provide such a cathodicsystem for irrigation piping have not been practical because of theunique requirement of the latter for mobility as well as economy andlong life.

It is therefore an object of my invention to provide an improvedcathodic system particularly adaptable for preventing corrosion intransportable piping systems.

A more specific object of the present invention is to provide a cathodicdevice for piping systems that may be contained within the piping butaccessible for servicing from outside the pipe at predeterminedlocations. This is an important advantage because it locates thecathodic protective system Where it can be most effective and yet whereit cannot be damaged when the pipe is handled over and over again duringits normal life. In general, my corrosion prevention system comprisesone or more sacrificial anodic wires each of which is supported withinthe pipe by an external member that may be helically wound on the anodicwire. The latter is anchored at its opposite ends at two spaced apartlocations of each pipe section, and these connections, protected byfluid tight seals, are accessible from outside the pipe when servicingof the anode is necessary. Thus, the present invention solves theproblem of providing an anodic device that is supported internallywithin the pipe, one that will provide adequate protection againstcorrosion for the entire pipe structure for long periods, and yet onewhich is easy to install and replace.

Another object of the present invention is to provide an anodic devicefor protecting pipes from corrosion originating from within the pipethat is particularly well adapted for ease and economy of manufacture.

Other objects, advantages and features of the invention will becomeapparent from the following detailed description thereof presented withthe accompanying drawings in which:

FIG. 1 is a view in elevation of a typical pipe section with portionsbroken away to show an anodic device installed therein according to theprinciples of the present invention;

FIG. 2 is an enlarged view in section showing details of one end of theinstalled anodic device of FIG. 1;

FIG. 3 is an enlarged view in section showing an alternate form ofanodic device according to my invention; and

FIG. 4 is a fragmentary view in perspective showing a pipe sectionhaving a pair of anodic devices according to my invention.

With reference to the drawing, FIG. 1 shOWs a section of irrigation pipe10 of the type that is generally installed on the surface of the groundfor sprinkling systems and the like so that it can be readily moved todifferent locations when required, Usually such pipe is made from asuitable aluminum alloy which, if unprotected, is vulnerable tocorrosion due to the electrolytic effect of impurities in the waterwithin the pipe. My cathodic device for providing the necessarycorrosion protection comprises generally an elongated device 11 whichextends within the pipe section from near one of its ends to near itsother end for substantially the full lentgh of the pipe. With the pipesection made of aluminum, the device 11 must contain a material that isanodic with respect to it. Thus, a portion of the device 11 isessentially a wire 12 of pure zinc. Since the wire 12 must extend foralmost the full length of the pipe section, the problem arose ofproviding it with adequate support and of maintaining electricalcontinuity through it as it is consumed as a sacrificial anode. Thisproblem is solved in the embodiment of FIGS. 1 and 2 by hehcallywrapping the anodic wire 12 with a supporting wire 13 of a strongermetal such as steel. For example, I have found that steel wire of 16gauge wrapped three turns per foot around an anodic zinc wire of 4;"diameter provldes suificient support to the anodic wire 12 and enables11 last for a long period of use even under severe environmentalconditions. Yet the two-wire combination of the sacrificial anodic wire12 and a neutral but conduct1ve supporting wire 13 still is easy tohandle during its installation within the pipe.

To secure the cathodic device 11 within the pipe sectlon as shown inFIG. 2, an anchoring means 14 is provided in combination with a circularopening 15 in the pipe wall near one end of the pipe. It includesmachine bolt 16 having a hex head 17 which extends through the openmg 15and into the pipe 10. The opening around the shank of the bolt 16 issealed by a first washer 18 which may be made of any slightly yieldablematerial, such as rubber. A second, fairly large and more rigid washer19 within the pipe 10 fits over the bolt shank and over the anodicdevice 11 looped around the shank. It is secured by a hex nut 20', thewasher 24 thus holding the anodic device firmly against the inner pipewall to assure good electrical contact. A similar attaching means 14a isprovided for securing the other end of the anodic device 11 with thepipe.

Since the attaching bolts 16 are located near the ends of the pipesection 10, the anodic wire 12 may be easily replaced by merelydisconnecting the pipe section temporarily and gaining access to the nut20 from the end of the pipe. Under average conditions, the anodic zincwire 12 will last from six to ten years when supported within the pipeby the wire 13, so the amount of servicing required is minimal.

As shown in FIG, 3 an alternate form of anodic wire may be used inaccordance with the present invention. Here, I provide a composite wire22 comprising a central core 23 of relatively stiff material for thenecessary strength such as steel wire. Surrounding this core 23 is anouter layer or coating 24 of expandable material such as zinc that againprovides the anode for my device. This wire may be connected in the sameway to the inside of the aluminum pipe, as described above, by wrappingits ends around bolts, such as the bolt 16, that extend inwardly fromthe sides of the pipe.

An anodic wire 12 or 22 installed according to my invention is efiectiveto prevent a corrosive attack on the pipe which extends for aconsiderable area on both sides of the wire. Thus, it is not necessaryfor the wire to extend absolutely to the end of the pipe section, asshown in FIG. 1. In pipe up to 6" in diameter, I have found that one ofmy elongated anode devices within the pipe is sufiicient, but for pipe6" to 10"- in diameter, two anode devices spaced 180 apart should beinstalled. In FIG. 4 is shown a typical arrangement Within such a pipe25 wherein two such anodic devices 26 and 27 are installed in such amanner. In pipe 12" in diameter and larger, three anode devices spacedapart should be used for maximum corrosion protection.

To those skilled in the art to which this invention relates, manychanges in construction and widely differing embodiments andapplications of the invention will suggest themselves without departingfrom the spirit and scope of the invention. The disclosures and thedescription herein are purely illustrative and are not intended to be inany sense limiting.

I claim:

1. -A cathodic protection system for preventing corrosion to an aluminumpipe section comprising: a wire extending longitudinally, within saidpipe section, said wire being of zinc material that is anodic withrespect to the aluminum material of said pipe; and anchoring meansattached to the ends of said wire for securing only said ends to theinside pipe wall near the opposite ends of said pipe section.

2. The system as described in claim 1 including supporting wire meanshelically wrapped around said zinc wire along its length.

3. The system as described in claim 2 wherein each said anchoring meansincludes a bolt having a head outside said pipe and a threaded shankextending inwardly through an opening in the pipe, sealing means betweensaid bolt head and the outer pipe wall around said opening and a nutthreaded on said bolt shank within the pipe for holding said zinc wirefirmly against the inside wall surface of the pipe.

4. A cathodic protection system for preventing corrosion to a section ofaluminum pipe, comprising: a plurality of sacrificial wires extendinglongitudinally within the pipe and spaced apart equally along its innersurface, said wires being of zinc and therefore anodic with respect tothe aluminum pipe material; supporting wire means helically wrappedaround each of said sacrificial zinc wires; and means extending throughthe wall of said pipe section at both ends for securing the ends of thesacrificial wires to the inner pipe surface.

References Cited UNITED STATES PATENTS 801,489 10/ 1905 Uthemann 204-1972,190,824 2/ 1940 Cook 204-197 2,619,455 11/1952 Harris et a1 204-1972,765,273 10/ 1956 Lobos 204197 3,037,926 6/1962 Ambler 204l97 3,109,51011/1963 Phelan 138--103 3,232,857 2/ 1966 Caldwell 204197 OTHERREFERENCES Fahrney et al., Chemical & Metallurgical Engineering, July1942, pp. 86 and 87.

JOHN H. MACK, Primary Examiner.

T. TUNG, Assistant Examiner.

U.S. Cl. X.R. 138-103

